Can't tell, is there glass in there? Is it the same thread on both sides or is it reversed on one? It could be a reversing ring to mount a lens backwards for macro photography.
–
ItaiApr 22 '13 at 21:52

There is no glass, and the threads go the same direction.
–
cyanosApr 29 '13 at 19:07

1 Answer
1

This looks like a spacer ring, which is used to add space between a lens and a filter (such as close-up filter) or other lens attachment to assist in focusing adjustment or increasing the angle of view.

Note, however, that using this could create some vignetting, especially on wide-angle lenses.